Linerless labels are known to have advantages over conventional pressure sensitive labels, which are mounted on a disposable, release-coated liner. The release-coated liner serves as a support structure for the transport, printing, and storage of conventional labels. In contrast, a linerless label has a release-coated top face and a pressure sensitive adhesive-coated bottom face, each applied to a paper substrate.
Linerless labels are wound in a roll con-figuration such that the pressure sensitive adhesive bottom face is in contact with the release coated top face. The presence of the release coating on a side opposite the pressure sensitive adhesive enables the labels to be wound to form a roll, and then subsequently unwound, without the adhesive sticking to the release coating. The adhesive further prevents the wound labels from unrolling prematurely.
Conventional methods of manufacturing linerless labels are disclosed in, for example, U.S. Pat. Nos. 5,292,713 and 5,354,588. An exemplary method includes the continuous steps of passing a thermally sensitive face paper through a coating machine and sequentially applying a layer of each of the various materials (e.g., release coating, and pressure sensitive adhesive) to the paper substrate, drying/curing the materials, and, thereafter, rolling the formed linerless labels into a roll, wherein the release-coated face on the outside of the rolled labels.
According to U.S. Pat. No. 5,292,713, suitable release coatings include materials that exhibit low adhesion to the pressure sensitive adhesive such that the formed label roll can be unrolled easily (e.g., without tearing the paper substrate, and without having the release coating stick to the pressure sensitive adhesive). An example of such a release coating composition contains 3 percent, by weight (wt. %) of the composition, of a photoinitiator based on the total weight of the composition, and 97 wt. % of an ultraviolet (UV) curable silicone based on the total weight of the composition. Both the photoinitiator and the curable silicone are commercially available, for example, under the tradenames "UV-9380C," and "UV-.sub.9300," respectively, from GE Silicones of Waterford, N.Y.
Linerless labels offer significant advantages over the conventional, liner-backed labels, most notably the elimination of the liner. Nonetheless, linerless labels have met only limited commercial success because such labels suffer from several disadvantages. For example, the pressure sensitive adhesives used in the manufacture of linerless labels are weak, and fail to adhere to the paper substrate, but rather preferentially adhere to the release coating. Furthermore, certain pressure sensitive adhesives can detrimentally effect the paper substrate under certain environmental conditions over time. Attempts to overcome these and other problems have focused on the use of additional tie layers that enhance bond strength between the release coating, substrate, and adhesive layers. Other attempts have been directed to modifying the chemical composition of the various layers.
Investigators previously attempted to resolve one or more of the aforementioned problems associated with linerless labels, but were relatively unsuccessful. One attempt utilized a formulation containing: (a) 57 to 80 wt. % of a curable epoxy silicone compound; (b) 12 to 32 wt. % of isopropyl alcohol; (c) 6 to 8 wt. % of an untreated silica powder; and, (d) 2 to 3 wt. % of a photoinitiator. However, unacceptably high gloss (i.e., a gloss value of greater than 8.2 at 20.degree. using a Technidyne gloss meter) resulted from formulations having 6 wt. % or less untreated silica powder.
In order to reduce gloss, the amount of silica was increased. But, the use of higher amounts of the untreated silica powder resulted in a viscous coating that was difficult to coat. Additional isopropyl alcohol was added to reduce composition viscosity, but the increase in the amount of untreated silica powder in the composition, coupled with the reduction in amount of epoxy silicone compound, resulted in a cured coating having an unacceptable amount of dusting attributed to poorly anchored silica powder. This surplus silica dust accumulates on the thermal printheads, thereby requiring cleaning of the printheads on a relatively continual basis. To help reduce the dust problem, the labels were perforated. However, perforations add to the cost of manufacturing and are not desirable in many applications.
It would be desirable to provide a release coating that overcomes one or more of the problems associated with linerless labels. More specifically, it would be desirable to provide a release coating that: (a) does not adhere to the pressure sensitive adhesive; (b) does not significantly insulate the underlying thermal sensitive layer; and (c) is of a suitable quality (e.g., of low gloss) such that the bar codes can be read via electronic means. Furthermore, it would be desirable to provide a release coating that exhibits sufficiently low spectral gloss such that the indicia imaged on the linerless label below the release coating can be read easily by suitable electronic means. Still further, it would be desirable to provide a low-gloss release coating that exhibits improved rub resistance, and provides better press stability during the process of manufacturing the linerless labels and eliminates the need to perforate the label.